The field of the invention is illumination and the invention is particularly concerned with a tri-spherical lens assembly for marine use.
Earlier designs of marine navigation lights were in two forms: One consisted of a fixture with a single continuous lens, the other of a "combined" fixture with a single light source and two or three separate, usually colored, lenses either joined directly together or separated by a relatively thin opaque vertical divider.
The fixture with the continuous lens, by virtue of its uninterrupted arc of visibility, could be designed to emit light over any predetermined horizontal arc simply by adjusting the angle subtended by the lens segment. The combined fixture, however, contains regions at the lens segment junctions that obscure transmission of light at those points. These designs also employed lens dividers, which comprise thin vertical screens projecting radially outward from the junction, that prevented the light emitted from one lens segment from crossing over into the region of visibility of the other lens segment. The crossing was caused by the lamp filament design, which was generally oriented horizontal and perpendicular to the vertical plane passing through the lens junction and the center of the filament.
According to the current state of the art a problem arises with the use of a single vertical filament. This is particularly true with a C-8 filament. This filament orientation is preferred in order to improve the cut-off (reduction of light emission at the ends of the horizontal prescribed arc of visibility) of the fixture. Using this technique to improve cut-off creates the undesirable obscuration at the junctions of the lens segments. The horizontally broad light source no longer exists to negate the obscuring effect of the vertical junctions. It is necessary, then, to control the transmission of light through the respective lens segments in the vicinity of the junctions in order to replace the light obscured by the junction.